Old school but it works.
Try looking a little bit away from where the noted traffic is supposed to be.
Vision, when it comes to moving objects is a bit better when a bit off axis.
Takes a little time working on it but if you become more aware of your peripheral vision you'll notice an improvement in picking up other aircraft.
Used to work on this by watching the television but actually looking at another object 40-50 degrees away from the screen. Takes a while but soon you'll know everything that is on the screen without looking at it directly.